This invention relates to radiological well logging methods and apparatus for investigating the characteristics of subsurface earth formations traversed by a borehole, and more particularly, to methods and apparatus for simultaneously measuring the porosity and thermal neutron capture cross section of earth formations in the vicinity of a well borehole by means of pulsed neutron well logging techniques.
In the search for hydrocarbons beneath the earth's crust one of the parameters which must be known about an earth formation before evaluating its commercial potential is the fractional volume of fluid filled pore space, or porosity, present around the rock grains comprising the earth formation. Several techniques have been developed in the prior art to measure earth formation porosity in a borehole environment. One such technique employs a gamma ray source and a single, or multiple, detectors to measure the electron density of the earth formations by gamma ray scattering. This leads to an inferential measurement of the porosity of the formations. Another technique employs an acoustic transmitter and one or more acoustic receivers. The velocity of sound transmission through the formation from the acoustic transmitter to the receivers is then measured and this quantity can be related to the porosity since sound travels faster in less porous rocks than in fluid filled pore spaces in the earth formations.
A third commercial technique which has been employed in the prior art to measure the porosity of earth formations employ a neutron source and either a neutron or gamma ray detector sensitive to low energy, or thermalized, neutron density. Hydrogen is the principal agent responsible for slowing down neutrons emitted into an earth formation. Therefore, in a formation containing a larger amount of hydrogen than is present in low porosity formations, the neutron distribution is more rapidly slowed down and is contained in the area of the formation near the source. Hence, the counting rates in remote thermal neutron sensitive detectors located several inches or more from the source will be suppressed. In lower porosity formations which contain little hydrogen, the source neutrons are able to penetrate farther. Hence, the counting rates in the detector or detectors are increased. This behavior may be directly quantified into a measurement of the porosity via well established procedures.
All of these commercially employed porosity measurement methods have generally not proven to be as accurate as desirable due to diameter irregularities of the borehole wall, variation of the properties of different borehole fluids, the irregular cement annulus surrounding the casing in a cased well borehole, and the properties of different types of steel casings and formation lithologies which surround the borehole. For example, the thermal neutron distribution surrounding a source and detector pair sonde as proposed in the prior art can be affected by the chlorine content of the borehole fluid. Similarly, lithological properties of the earth formations in the vicinity of the borehole, such as the boron content of these formations, can affect the measurement of thermal neutron populations. The present invention however, rather than relying on a measurement of the thermal neutron population for porosity comprises a neutron measurement of the formation porosity which utilizes a measurement of the epithermal neutron population at one detector and the fast neutron population at a second detector.
Perhaps the most important parameter necessary for commercial interpretation of a prospective formation is the water saturation Sw of the zone. In the prior art the most prevalent technique for interpreting the water saturation of a zone in a cased well borehole has been by means of the thermal neutron capture cross section .SIGMA., of the zone. In zones filled with oil and salt water, thermal neutron decay time, thermal neutron lifetime, or thermal neutron die away logs have been successfully used to measure the macroscopic thermal neutron capture cross section of the formations. The water saturation Sw is related to the porosity .phi. and the thermal neutron capture cross section .SIGMA. by the standard relationship ##EQU1## where .SIGMA..sub.MA = thermal neutron capture cross section of the rock matrix.
.SIGMA..sub.HC = thermal neutron capture cross section of oil (hydrocarbon) PA1 .SIGMA..sub.SH = thermal neutron capture cross section of shale PA1 V.sub.sh = volume shale fraction of the formation PA1 .SIGMA. = total measure capture cross section PA1 .phi. = total measured porosity.
In the prior art two separate logging passes have generally been necessary to accurately measure .phi. and .SIGMA.. This is due to the fact that in the prior art, porosity logging devices using 14 MEV neutron sources (which are presently required in all .SIGMA. logging devices) have had to rely on inaccurate measurements utilizing thermal neutron capture gamma rays to determine the porosity. As previously mentioned, in the present invention fast and epithermal neutron energy measurements are used to determine the porosity .phi., while thermal neutron capture gamma rays are measured only to determine the capture cross section .SIGMA.. This is done in one pass of the logging instrument, thus saving costly rig time and providing a much speedier and more accurate measurement than heretofore has been possible.
Thus, it is an object of the present invention to provide an improved method and apparatus for simultaneously measuring the porosity and thermal neutron capture cross section of earth formations in situ in a well borehole using pulsed neutron source techniques.